1. Field of the Invention:
This invention relates to nuclear reactors, and more particularly provides an insulation and preheat system for selected reactor components within which circulates a fluid coolant.
2. Description of the Prior Art:
In a nuclear reactor, and in the particular exemplary liquid metal cooled nuclear reactor, components through which the liquid metal circulates must be insulated. Insulation is required to minimize waste of thermal power generated in the reactor, to limit the heating of adjacent structures such as concrete and steel supports, and to allow the components to be maintained above the freezing point of the liquid metal coolant during reactor shutdown. Presently, close-fitting thermal insulation is typically installed directly onto the components. For example, insulating material contained between two stainless steel sheets is attached about the components by steel bands. In order to maintain the components above the coolant freezing temperature, resistance heaters or heat tracing are installed in an annular region, typically about one inch in depth, between the insulation and the component surface. In order to limit heating of supporting structures, a cooled nitrogen atmosphere is typically circulated through a concrete cell structure surrounding the insulated components. A similar prior art preheat and cooling system is described in U.S. Pat. No. 3,155,595, in the name of D. C. Schluderberg.
Although these present systems adequately perform the required functions, they are not without deficiencies. Foremost, as a result of the stringent regulatory and industrial safety oriented inspection requirements imposed upon nuclear facilities, periodic in-service inspections are performed. These include routinely scheduled visual inspection of the outer surface of accessible reactor components. It will be readily apparent that the close fitting insulation systems presently in use make such inspections extremely difficult and time consuming. Lack of inspectability of certain components due to the presence of the close-fitting thermal insulation may prove unacceptable in the future.
Further, the capital cost associated with present systems are high. A large number of electrical trace heaters are required in order to distribute the heating evenly over the insulated components and thereby minimize thermal stresses during preheating. This further requires a large amount of electrical circuitry, including wires, penetrations, junction boxes, switchgear, and so forth, to power and control the trace heaters. Further, a large number of automatic temperature controllers and thermocouples are needed to regulate the heatup of the components.
Additionally, should the heaters burn out or require maintenance, they are not readily accessible. Also, connecting auxiliary components are typically insulated and trace heated separately, further complicating the system.
It therefore is highly advantageous in terms of safety, cost, time and ease of maintenance, to provide an insulation and preheat system which overcomes the above-listed deficiencies.